The speaker was getting ready to put his condo in DC on the market and thought that he can get a lot of money from the sale. He then referred to his condo as the “golden goose egg.” This is an incongruent conflation with opposite meanings – “kill the goose that laid the golden egg” (destroy riches through stupidity), “golden egg” (rich) and “goose egg” (zero). “Sitting on a gold mine” (having something very valuable) is probably the expression the speaker meant to say, but then that big golden goose appeared in his mind and the rest was conflation. This one is good as gold, and my thanks to Sid Sher for sending it in!